1. Field of the Invention
The present invention relates to a process for producing lead frames for semiconductor devices, the lead frames having extremely finely patterned lead tips.
2. Description of Related Art
The increasing degree of integration in semiconductor chips requires lead frames having lead tips arranged at narrower intervals than ever before so that it can provide a large number of pins. (The lead frame connects the semiconductor chip to external circuits for exchanging electrical signals and permits the mounting of the semiconductor chip.) There are two methods available now for producing lead frames--stamping with a die and pattern etching with a photoresist.
Stamping is suitable for mass production once a die is completed, whereas etching is suitable for production of a variety of products, each in small quantities, because it permits design to be made or changed in a short time. Another advantage of etching is its capability of fine patterning. Unfortunately, etching is subject to so-called side etching, which is a phenomenon that etching takes place not only in the direction of the thickness of the lead frame substrate but also in the direction perpendicular to the thickness. The consequence of side etching is that finely patterned lead frame tips have a flat width narrower than designed. Therefore, the current lead frame produced by etching has tips as shown in FIG. 2. In the case of a lead frame having a pitch (.alpha.) of 170 .mu.m, the accuracy of finishing is limited to 75 .mu.m for the flat width (.beta.) and 80 .mu.m for the lead space (.gamma.).